Malathion was evaluated by the JMPR in 1963, 1965, and 1966. An ADI at 0-0.02 mg/kg bw was established in 1963, and confirmed in 1965 and 1966. Malathion is now evaluated in the CCPR periodic review programme.

Malathion is rapidly absorbed, biotransformed, and excreted, predominantly in the urine but also in the faeces, largely as its two monocarboxylic acids and the dicarboxylic acid.

The oral LD50 values for malathion in laboratory rodents were 1000-10,000 mg/kg bw, the observed differences probably being due to impurities. The most recent LD50 values tend to be higher. The cholinesterase-inhibiting metabolite of malathion, malaoxon, has much lower oral LD50 values of 100-220 mg/kg bw. WHO has classified malathion as slightly hazardous.

In a study of acute neurotoxicity in rats receiving doses of 0, 500, 1000, or 2000 mg/kg bw, an NOAEL was not identified, as clinical signs were present at all doses. In a 13-week study of neurotoxicity, also in rats, at dietary concentrations of 0, 50, 5000, or 20,000 ppm, the NOAEL was 5000 ppm, equal to 350 mg/kg bw per day, on the basis of inhibition of brain acetylcholinesterase at the highest dose.

In a 30-day study of toxicity in rats receiving malathion in the diet at concentrations of 0, 50, 100, 500, 10,000, or 20,000 ppm, the NOAEL was 500 ppm, equal to 52 mg/kg bw per day, on the basis of increased liver weight and histopathological changes in the liver (periportal hepatocyte hypertrophy) at the next highest dose.

In a 90-day study of toxicity in rats, malathion was given at dietary concentrations of 0, 100, 500, 5000, 10,000, or 20,000 ppm. The NOAEL was 500 ppm, equal to 34 mg/kg bw per day, on the basis of decreased mean corpuscular volume and mean corpuscular haemoglobin, increased liver weights and relative kidney weights, and chronic nephropathy in males, and decreased mean cell volume, hepatocyte hypertrophy, and increased relative kidney weight in females at the next highest dose.

A 21-day study of dermal toxicity with malathion at doses of 0, 50, 300, or 1000 mg/kg bw per day, 6 h per day, five days per week, was carried out in rabbits. The NOAEL was 300 mg/kg bw per day on the basis of inhibition of brain acetylcholinesterase activity at the highest dose.

In a 28-day study of toxicity in dogs, malathion was fed in gelatin capsules at doses of 0, 125, 250, or 500 mg/kg bw per day for 28 consecutive days. An NOAEL was not identified because of clinical signs at all doses.

In a one-year study of toxicity in dogs, malathion was administered orally in capsules at doses of 0, 62.5, 125, or 250 mg/kg bw per day on seven days per week. The NOAEL was 125 mg/kg bw per day on the basis of body-weight depression and changes in haematological and clinical chemical parameters at the highest dose.

A number of long-term studies of toxicity and carcinogenicity have been carried out on malathion in both rats and mice. The earlier ones were reviewed by IARC, which concluded that the available data did not provide evidence that malathion was carcinogenic.

In an 18-month study in mice, malathion was administered at dietary concentrations of 0, 100, 800, 8000, or 16,000 ppm. The NOAEL was 800 ppm, equal to 140 mg/kg bw per day, on the basis of inhibition of brain acetylcholinesterase activity at termination and an increased incidence of liver adenomas in animals of each sex at the next highest dose.

In a two-year study in rats, dietary concentrations of 0, 100, 1000, or 5000 ppm were used. The NOAEL was 100 ppm, equivalent to 5 mg/kg bw per day, on the basis of reduced erythrocyte acetylcholinesterase activity and body weight. In another long-term study in rats, malathion was given at doses of 0, 100/50, 500, 6000, or 12,000 ppm for two years. The NOAEL was 500 ppm, equal to 29 mg/kg bw per day, on the basis of decreased survival and body-weight gain, changes in haematological parameters, decreased brain acetylcholinesterase activity, increased g -glutamyl transpeptidase activity, increased liver, kidney, and thyroid/parathyroid weights, and changes in the olfactory epithelium at the next highest dose.

A number of studies of reproductive toxicity have been carried out, only some of which provided NOAELs. In a study in rats, malathion was administered by gavage to groups of pregnant animals on days 6-15 of gestation at doses of 0, 200, 400, or 800 mg/kg bw per day. The NOAEL was 400 mg/kg bw per day on the basis of maternal toxicity at the highest dose; no fetal toxicity was observed.

Malathion was administered orally at doses of 0, 25, 50, or 100 mg/kg bw per day to groups of pregnant rabbits on days 6-18 of gestation. The NOAELs were 25 mg/kg bw per day for maternal toxicity and 100 mg/kg bw per day for fetal toxicity; teratogenicity was not seen at any dose.

A two-generation study was undertaken in rats in which malathion was given at dietary concentrations of 0, 550, 1700, 5000, or 7500 ppm. The NOAEL was 7500 ppm, equal to 600 mg/kg bw per day, for reproductive toxicity and 1700 ppm, equal to 130 mg/kg bw per day, for developmental toxicity, the latter being based on reduced pup weights.

Numerous tests have been carried out for genotoxicity both in vitro and in vivo. Most of the evidence indicates that malathion is not genotoxic, although there is some evidence that it can produce chromosomal aberrations and sister chromatid exchange in vitro. There was no evidence that malathion induces chromosomal aberrations in vivo. Malaoxon did not induce reverse mutation in bacteria, but it caused sister chromatid exchange in two tests in mammalian cells and induced sex-linked recessive lethal mutation in Drosophila in vivo. The four common impurities of malathion, isomalathion, O,O,S-trimethylphosphorothioate, O,S,S-trimethyl phosphorodithioate, and O,O,O-trimethyl phosphorothioate, did not induce reverse mutation in bacteria. The Meeting concluded that malathion is not genotoxic.

Two studies on the neurotoxicity of malathion in hens were reviewed. In neither was there evidence that malathion can cause delayed neuropathy, although some inhibition of neuropathy target esterase was found in the brain at 2000 mg/kg bw.

In a study in volunteers with doses of 8, 16, or 24 mg of malathion per day, the NOAEL was 16 mg per day (equivalent to 0.27 mg/kg bw per day) on the basis of inhibition of plasma and erythrocyte cholinesterase activity. Several cases of exposure to impure malathion have been reported, none of which resulted in delayed neuropathy.

An ADI of 0-0.3 mg/kg bw was established on the basis of the NOAEL of 29 mg/kg bw per day in the two-year study of toxicity and carcinogenicity in rats using a safety factor of 100. This ADI is supported by the NOAEL of 25 mg/kg bw per day in the study of developmental toxicity in rabbits. The alternative approach of basing the ADI on the study in humans was not taken, as the study was old and the material was therefore likely to contain toxic impurities.

A toxicological monograph was prepared.

TOXICOLOGICAL EVALUATION

Levels that cause no toxic effect

Mouse:

800 ppm, equal to 140 mg/kg bw per day (18-month study of toxicity and carcinogenicity)

Rat:

500 ppm, equal to 29 mg/kg bw per day (two-year study of toxicity and carcinogenicity)

400 mg/kg bw per day (maternal toxicity in a study of developmental toxicity)

1700 ppm, equal to 130 mg/kg bw per day (study of reproductive toxicity)

Rabbit:

25 mg/kg bw per day (maternal toxicity in a study of developmental toxicity)

Dog:

130 mg/kg bw per day (one-year study of toxicity)

Human:

0.3 mg/kg bw per day (47-day study of toxicity)

Estimate of acceptable daily intake for humans

0-0.3 mg/kg bw

Studies that would provide information useful for the continued evaluation of the compound

Further observations in humans.

Toxicological criteria for setting guidance values for dietary and non-dietary exposure to malathion